The present invention relates to a white balance control circuit for a projection television receiver, and more particularly to a white balance control circuit for automatically controlling balance of color when quantities of light of the three primary colors decrease with time.
In recent years, in order to provide a wider screen for a television receiver, it has become common to employ a projection television system.
A rear projection system which is one of the systems of the projection television is shown in FIG. 5. A picture tube 1 produces a pictorial image which is expanded by a lens 2. The image is reflected on opposing reflecting mirrors 3 and 4 and reproduced on a light-transmissible screen 5.
FIG. 6 shows another projection television receiver where the image is reproduced on a reflecting screen 6 from the front thereof.
There are other types of projection system such as a refraction system shown in FIG. 7 and a reflection system shown in FIG. 8 dependent on the type of picture tubes 1. In the refraction system, the lens 2 is provided in front of a fluorescent screen of the picture tube 1. The lens 2 expands and reproduces the image on the reflecting screen 6.
To the contrary, in the reflection system, a picture tube 8 has a fluorescent screen 9 and a reflecting mirror 10 provided in the tube 8. The image reflected on the reflecting mirror 10 is expanded through a Schmidt lens 11 and reproduced on the reflecting screen 6.
In accordance with the rear projection system shown in FIG. 5, since the picture tube 1 and the lens 2 can be disposed adjacent the screen 6, the television receiver can be reduced in size. The light-transmissible screen 5 is made of a material having a high light transmittance, such as a Fresnel lens and a lenticular sheet.
In the projection color television receiver, there are three picture tubes 1 (8) for red (R), green (G) and blue (B). White balance is obtained by adjusting ratio of light between the tubes into red light of 30%, green light of 59%, and blue light of 11%. However, the fluorescent screen 7 (9) of the picture tube has a low luminous efficacy particularly for blue. Accordingly, in order to maintain the balance with the other colors, current i supplied to the blue picture tube is increased. Thus the joule heat caused by the current i heats the screen of the blue picture tube, thereby quickly deteriorating it.
Hence, as the screen of the blue picture tube deteriorates with time, the luminous efficacy of the blue light is further decreased. As a result, the quantities of red and green lights relatively increases. The picture on the screen 6 will thus gradually assume a yellowish color. The color can be corrected by manipulating a dial for controlling the color balance. However, the adjusting operation is complicated.